This invention is concerned with an improvement in a process for the production of block copolymers.
The block copolymers considered are synthesized by forming a first polymer block of a monoalkenylarene and thereafter block polymerizing a conjugated diene therewith. The process employs a lithium-based initiator and the polymerization is conducted in solvents more fully discussed hereinafter.
After the formation of the first two polymer blocks a number of options exist, including termination of the polymerization, additional block formation or coupling to obtain linear or branched block copolymer configurations.
It has been found that the rate of reactivity of monoalkenylarene with lithium-based polymerization initiators is substantially lowered in the presense of conjugated dienes or other active unsaturated compounds, such as acetylenes. Consequently, if the solvent employed in the block polymerization contains any of these relatively active species, these not only will consume part of the extremely small amount of lithium-based initiator which is present in the system, but also will cause a "skewing", by which is meant the irregular or erratic initiation of polymerization of the monoalkenylarene. This results in the production of a first polymer block having an undesirably broad molecular weight spectrum. Furthermore, due to consumption of an indeterminate amount of the lithium-based initiator, the resulting arene polymer block will have an undesirably high average molecular weight which may vary from batch to batch. These trace impurities also may cause other problems, including gel production and undesirable polymer microstructure.